Cross-sectionally and Longitudinally Balanced Effects of Processing Speed on Intellectual Abilities

The processing speed theory of cognitive aging states that declines in intellectual abilities other than processing speed are mainly due to a slowing of processing speed. Cross-sectional studies have provided support for the processing speed theory as for age-related cognitive differences. Longitudinal studies, in turn, have provided weaker support for the processing speed theory as for cognitive age changes. The present study aims to reconcile this discrepancy of cross-sectional and longitudinal results by constructing a "fair" test of the processing speed theory, i.e., a test that balances the cross-sectional age range and the time period covered longitudinally. Data from 83 older adults came from the Bonn Longitudinal Study on Aging. Using hierarchical linear models, it is shown that, although cross-sectional age differences and longitudinal changes in the intellectual abilities were equal, processing speed only attenuated cross-sectional age effects in cognition but virtually did not reduce longitudinal change effects in cognition. This persisting difference in the explanatory power of processing speed regarding age-related differences and age-related changes is discussed with reference to other longitudinal studies and statistical issues regarding cross-sectional and longitudinal studies of aging.     

Age-related slowing of digit symbol substitution revisited: what do longitudinal age changes reflect?

A previous investigation reported that cross-sectional age differences in Digit Symbol Substitution (DSS) test performance reflect declines in perceptual processing speed. Support for the tenability of the processing speed hypothesis requires examining whether longitudinal age-related change in DSS performance is largely mediated by changes in speed. The present study used data from the Victoria Longitudinal Study to examine patterns and predictors of longitudinal change in DSS for 512 older adults (M(age) = 68.37 years, SD = 7.43). On the basis of multilevel modeling, baseline DSS performance was poorer for older participants and men, with longitudinal declines more pronounced with increasing age and decreasing speed. In contrast to the present cross-sectional findings, statistical control of change trajectories in perceptual speed using the same data did not substantially attenuate age changes. These discrepancies suggest different sources of variance may underlie cross-sectional age differences and longitudinal age changes for DSS.     

Cognitive speed and subsequent intellectual development: a longitudinal investigation.

The hypothesis that a measure of intellectual speed assessed at one point in time would predict intellectual achievement at a later point in time was evaluated with a time-lagged cross-correlational analysis, an application of causal modeling techniques. Longitudinal data for 32 males and females, tested in 1944 (mean age 19.5 years) and in 1972 (mean age 46.7 years), supported the hypothesized relationships with an associated p less than .01. The Relations Factor of the Army Alpha Examination--consisting of scores from a highly speeded simple analogies test and a short-term memory test--administered at age 20 was highly predictive of both verbal and numerical ability in middle age. The results highlight the cognitive intellectual aspect of the speed of behavior. In addition, these findings supplement Hunt's studies of the relationships between speed of cognitive processing and psychometric abilities in young adults, and emphasize the importance of cognitive speed for subsequent intellectual development. Implications for the intellectual speed hypothesis of Birren and the utilization of time-lag designs in longitudinal research are discussed.     

Short-term versus long-term longitudinal changes in processing speed

BACKGROUND: Previous longitudinal studies of cognitive aging have focused on long-term performance changes. A recent surge of research has demonstrated that there are reliable interindividual differences in short-term cognitive performance changes. OBJECTIVE: The present study links these two pathways of cognitive aging research by examining the association between short-term (learning, practice) versus long-term (development) changes in processing speed. METHODS: Data from 963 elderly participants come from the Longitudinal Aging Study Amsterdam (LASA). RESULTS: Nested latent growth curve analyses show that the amount of learning or practice in processing speed at first measurement occasion is positively related (r = 0.72) to individual differences in development of processing speed across 6 years. CONCLUSIONS: Short-term learning or practice gains in processing speed are positively associated with long-term developmental changes in processing speed in the elderly.     

Understanding Ageing. An evaluation of research designs for assessing the interdependence of ageing-related changes.

BACKGROUND: Cross-sectional studies of samples varying widely in age have found moderate to high levels of shared age-related variance among measures of cognitive and physiological capabilities, leading researchers to posit common factors or common causal influences for diverse age-related phenomenon. OBJECTIVE: The influence of population average changes with age on cross-sectional estimates of association has not been widely appreciated in developmental and ageing research. Covariances among age-related variables in cross-sectional studies are highly confounded in regards to inferences about associations among rates of change within individuals since covariances can result from a number of sources including average population age-related differences (fixed age effects) in addition to initial individual differences and individual differences in rates of ageing (random age effects). Analysis of narrow age-cohort samples may provide a superior analytical basis for testing hypotheses regarding associations between rates of change in cross-sectional studies. CONCLUSIONS: The use of age-heterogeneous cross-sectional designs for evaluating interdependence of ageing-related processes is discouraged since associations will not necessarily reflect individual-level correlated rates of change. Typical cross-sectional studies do not provide sufficient evidence for the interdependence of ageing-related changes and should not serve as the basis for theories and hypotheses of ageing. Reanalyzing existing cross-sectional studies using a sequential narrow-age cohort approach provides a useful alternative for evaluating associations between ageing-related changes. Longitudinal designs, however, provide a much stronger basis for inference regarding associations between rates of ageing within individuals.     

Age-related changes in human and non-human primate white matter: from myelination disturbances to cognitive decline

The cognitive decline associated with normal aging was long believed to be due primarily to decreased synaptic density and neuron loss. Recent studies in both humans and non-human primates have challenged this idea, pointing instead to disturbances in white matter (WM) including myelin damage. Here, we review both cross-sectional and longitudinal studies in humans and non-human primates that collectively support the hypothesis that WM disturbances increase with age starting at middle age in humans, that these disturbances contribute to age-related cognitive decline, and that age-related WM changes may occur as a result of free radical damage, degenerative changes in cells in the oligodendrocyte lineage, and changes in microenvironments within WM.     

Two-year decline in vision but not hearing is associated with memory decline in very old adults in a population-based sample

BACKGROUND: Recent cross-sectional research in cognitive aging has demonstrated a robust association between visual acuity, auditory thresholds and cognitive performance in old age. However, the nature of the association is still unclear, particularly with respect to whether sensory and cognitive function are causally related. OBJECTIVE: This study aimed to determine whether marked declines in performance on screening measures of either visual acuity or auditory thresholds have an effect on cognitive decline over 2 years. METHODS: The sample from the Australian Longitudinal Study of Ageing (n = 2,087) were assessed in 1992 and 1994 on measures of sensory and cognitive function as part of a larger clinical assessment. A quasi-experimental design involving comparison of extreme groups using repeated measures MANCOVA with age as a covariate was used. RESULTS: Group performance on measures of hearing, memory, verbal ability and processing speed declined significantly. Decline in visual acuity had a significant effect on memory decline, but not on decline in verbal ability or processing speed. Decline in hearing was not associated with decline in any cognitive domain. CONCLUSION: The common association between visual acuity, auditory thresholds and cognitive function observed in cross-sectional studies appears to be disassociated in longitudinal studies.     

It's never too late to engage in lifestyle activities: significant concurrent but not change relationships between lifestyle activities and cognitive speed

Little is known about potential longitudinal relationships between participation in social, physical, and intellectual activities and later cognitive performance. Data from the Victoria Longitudinal Study (n = 530) were used to test whether baseline and change in lifestyle engagement were related to corresponding indicators of cognitive speed (measured by mean-level and intraindividual variability). Regressions based on random effects model estimates showed that cross-sectional activity participation predicted corresponding values of both mean-level and intraindividual variability, but few longitudinal relationships were significant. Overall, a higher frequency of participation in cognitively complex activities was related to faster response times and lower intraindividual variability. Findings suggest that activity level at one point in time may be a more important predictor of cognition than an individual's changes in activity level.     

The interpretation of shared age-related variance among factors in cross-sectional cognitive aging studies

Many cross-sectional correlational studies in cognitive aging have focused on explaining age-related variance. It has been assumed that variables sharing variance with both cognition and age may be the key explanatory variables underlying the cognitive decline in normal aging. Statistical biases intrinsic to this approach have been described by Hofer and Sliwinski and a narrow age cohort design proposed. The present paper aims at explaining how Hofer and Sliwinski's criticisms apply to a specific type of research design in cognitive aging where the goal is to identify underlying aging processes, but does not apply to more general gerontological research. Methods to estimate bias in cross-sectional studies are required as is greater awareness of this potential bias.     

The relationship between longitudinal declines in dehydroepiandrosterone sulfate concentrations and cognitive performance in older men

BACKGROUND: The observation that dehydroepiandrosterone (DHEA) concentrations decrease markedly with age has led to the hypothesis that declining DHEA concentrations may contribute to age-related changes in cognition. In the United States, DHEA is widely available as an over-the-counter supplement that individuals are using in an effort to ameliorate age-related cognitive and physical changes. OBJECTIVE: To investigate the relationship between age-associated decreases in endogenous DHEA sulfate (DHEA-S) concentrations and declines in neuropsychological performance in a prospective, longitudinal study. METHODS: The subjects were 883 men from a community-dwelling volunteer sample in the Baltimore Longitudinal Study of Aging. The men were aged 22 to 91 years at the initial visit, and they were followed up for as long as 31 years (mean, 11. 55 years), with biennial reassessments of multiple cognitive domains and contemporaneous measurement of serum DHEA-S concentrations. Outcome measures were the results of cognitive tests of verbal and visual memory, 2 tests of mental status, phonemic and semantic word fluency tests, and measures of visuomotor scanning and attention. Serum DHEA-S concentrations were determined by standard radioimmunoassay. RESULTS: Neither the rates of decline in mean DHEA-S concentrations nor the mean DHEA-S concentrations within individuals were related to cognitive status or cognitive decline. A comparison between the highest and lowest DHEA-S quartiles revealed no cognitive differences, despite the fact that these groups differed in endogenous DHEA-S concentration by more than a factor of 4 for a mean duration of 12 years. CONCLUSION: Our longitudinal results augment those of previous prospective studies by suggesting that the decline in endogenous DHEA-S concentration is independent of cognitive status and cognitive decline in healthy aging men.     

A reevaluation of the common factor theory of shared variance among age, sensory function, and cognitive function in older adults

The common cause hypothesis of the relationship among age, sensory measures, and cognitive measures in very old adults was reevaluated. Both sensory function and processing speed were evaluated as mediators of the relationship between age and cognitive function. Cognitive function was a latent variable that comprised 3 factors including memory, speed, and verbal ability. The sample was population based and comprised very old adults (n = 894; mean age = 77.7, SD = 5.6 years) from the Australian Longitudinal Study of Ageing. The results showed that there was common variance in the cognitive factor shared by age, speed, vision, and hearing but that specific effects of age on cognition remained. Furthermore, speed did not fully mediate the effect of age or sensory function on cognition. Some age differences in cognitive performance are not explained by the same processes that explain age differences in sensory function and processing speed.     

Biomedical and social determinants of cognitive impairment in the elderly

Studies in age-related cognitive changes do not necessarily take into account the clinical and social phenomena associated with age that may have more effect on cognition than does psychologic senescence. Clinically inapparent biologic deterioration, drugs, mild dementia, and atypically presenting depression may influence cognition. Selection bias may skew the results of studies on the elderly when control groups are made up of undergraduates or of "normal" elderly whose normality is taken for granted rather than verified. The epiphenomena of aging, such as retirement, social isolation, and bereavement, which are not inherent in the aging process, influence cognition, as do labeling and learned helplessness. Laboratory-based tests of cognition may not sufficiently resemble real-life conditions to have validity. Future research will be shaped by how we conceptualize the relation between aging and intellectual function.     

Longitudinal evidence for diminished frontal cortex function in aging

Cross-sectional estimates of age-related changes in brain structure and function were compared with 6-y longitudinal estimates. The results indicated increased sensitivity of the longitudinal approach as well as qualitative differences. Critically, the cross-sectional analyses were suggestive of age-related frontal overrecruitment, whereas the longitudinal analyses revealed frontal underrecruitment with advancing age. The cross-sectional observation of overrecruitment reflected a select elderly sample. However, when followed over time, this sample showed reduced frontal recruitment. These findings dispute inferences of true age changes on the basis of age differences, hence challenging some contemporary models of neurocognitive aging, and demonstrate age-related decline in frontal brain volume as well as functional response.     

Aging and longitudinal change in perceptual-motor skill acquisition in healthy adults

Knowledge about aging of perceptual-motor skills is based almost exclusively on cross-sectional studies. We examined age-related changes in the retention of mirror-tracing skills in healthy adults who practiced for 3 separate days at baseline and retrained 5 years later at follow-up. Overall, the speed and accuracy of an acquired skill were partially retained after a 5-year interim, although the same asymptote was reached. Analyses with individual learning curves indicated that the effects of age on mirror-tracing speed were greater at longitudinal follow-up than at baseline, with older adults requiring more training to reach asymptote. Thus, although the long-term retention of acquired skills declines with age, older adults still retain the ability to learn the skill. Moreover, those who maintained a processing speed comparable with that of the younger participants evidenced no age-related performance decrements on the mirror-drawing task.     

Biological age and 12-year cognitive change in older adults: findings from the Victoria Longitudinal Study

BACKGROUND: Although recent cross-sectional findings indicate that markers of biological age (BA) mediate chronological age (CA) differences in cognitive performance, little is known about their influence on actual cognitive changes. OBJECTIVE: The purpose of this investigation is to examine CA and BA as predictors of 12-year cognitive change in a longitudinal sample of older adults. METHODS: Data from the Victoria Longitudinal Study (VLS) were examined for 125 adults between 67 and 95 years of age. Biomarkers, including visual and auditory acuity, grip strength, peak expiratory flow, blood pressure, and body mass index, were submitted to a factor analysis and a composite BA variable was computed based on factor loadings. Intraindividual change across 5 waves of measurement (3-year intervals) was examined as a function of CA and BA for 5 cognitive domains: verbal processing speed, working memory, reasoning, episodic memory, and semantic memory. RESULTS: The latent structure of biomarkers was consistent with previous investigations of functional age and a common factor view of biological aging. Results of hierarchical linear modeling showed that BA predicted actual cognitive change (decline) independent of CA. CONCLUSIONS: As a predictor of cognitive performance in late life, CA is a proxy for biological and environmental influences. We have shown that biological influences are independent predictors of actual cognitive change in older adults. This supports the view that cognitive decline is not due to aging per se, but rather is likely due to causal factors that operate along the age continuum.     

Aging, cohorts, and verbal ability

OBJECTIVES: Age-related differences in cognitive abilities observed in cross-sectional samples of individuals varying in age may in part be spurious due to the effects of cohort differences in schooling and related factors. This study examined the effects of aging on cognitive function controlling for any and all differences in cohort-based social experiences of different age groups. METHODS: We examined age-related patterns in a measure of verbal ability using 14 repeated cross-sectional surveys from the General Social Survey (GSS) over a 24-year period. RESULTS: The raw GSS data show the expected age-related growth and decline in vocabulary knowledge, but these age differences are reduced when adjusted for cohort differences. There is evidence of small age-related patterns in vocabulary knowledge within cohorts, but the curvilinear contributions of aging to variation in verbal scores account for less than one-third of 1% of the variance in vocabulary knowledge, once cohort is controlled. Cohort differences in schooling contribute substantially to this effect. DISCUSSION: Within-age-group variation in vocabulary knowledge is vastly more important than age differences per se, and the complexities of the relationship of verbal skills to historical differences in the experience of schooling present an interesting avenue for future research.     

Age-related disruptions of circadian rhythm and memory in the senescence-accelerated mouse (SAMP8)

Common complaints of the elderly involve impaired cognitive abilities, such as loss of memory and inability to attend. Although much research has been devoted to these cognitive impairments, other factors such as disrupted sleep patterns and increased daytime drowsiness may contribute indirectly to impaired cognitive abilities. Disrupted sleep–wake cycles may be the result of age-related changes to the internal (circadian) clock. In this article, we review recent research on aging and circadian rhythms with a focus on the senescence-accelerated mouse (SAM) as a model of aging. We explore some of the neurobiological mechanisms that appear to be responsible for our aging clock, and consider implications of this work for age-related changes in cognition.     

Age and ability affect practice gains in longitudinal studies of cognitive change

During a 20-year longitudinal study, 5,842 participants aged 49 to 93 years significantly improved over two to four successive experiences of the Heim AH4-1 intelligence test (first published in 1970), even with between-test intervals of 4 years and longer. After we considered significant attrition by death and dropout and the effects of gender, socioeconomic advantage, and recruitment cohort, we found that participants with high intelligence test scores showed greater improvement than did those with lower intelligence test scores. Practice gains also reduced with age, even after we took into consideration the individual differences in intelligence test scores. This emphasizes the methodological point that neglect of individual differences in improvement during longitudinal studies underestimates age-related changes in younger and more able participants and the theoretical point that, like all experiences during everyday life, participation in longitudinal studies alters the ability of aging humans to cope with cognitive demands to different extents according to their baseline abilities.     

Cross-sectional age differences and longitudinal change on the Bradburn Affect Balance Scale

Cross-sectional age differences and longitudinal change were examined on psychological well-being, positive affect, and negative affect, as measured by the Bradburn Affect Balance Scale. Data were collected from 1,159 participants in 1971 and 1985. Cross-sectional analyses showed age differences: older cohorts reported greater overall well-being but lower levels of both positive and negative affect when compared to younger respondents. Longitudinal analyses indicated small but significant changes toward decreased positive and negative affect but increased overall well-being. Negative affect had the strongest effect size. Positive and negative affect showed different patterns of change for different age groups. Taken together, cross-sectional and longitudinal findings suggest that change in affect variables is age-related, although these changes are relatively small. More evident was a pattern of correlational stability with age. Finally, the pattern of the results supports a two-factor theory of psychological well-being.     

Processing speed, attentional capacity, and age-related memory change

This study compared the relative importance (i.e., proportion of shared variance) of attentional capacity and processing speed accounts of cognitive aging to predict age differences in episodic and working memory performance. Right-handed adults (n = 100), 18 to 88 years of age, completed measures of attentional capacity (divided attention), processing speed, and episodic and working memory. The results provide little support for the predictive utility of the attentional capacity construct, independent of processing speed ability in accounting for age-specific episodic memory relations. The results are, however, consistent with the notion that attentional capacity mediates aspects of age-related working memory change.